Dot matrix printing is the general term for the production of a printed image upon a print medium accomplished by means of the depositing of a plurality of small individual spots or "dots" of ink upon the medium which, when viewed together, closely approximate the desired image. This general method is commonly used in the production of a variety of printed images and illustrations. It is effective because the printed dots can be made to be closer to or further away from each other to produce more or less darkly shaded areas as desired. Further, the phenomena of human perception tend to cause viewers to perceive the overall image that is intended, in spite of minor irregularities. Within the limits imposed by the relative size of the smallest portions of the image to be imprinted (the "detail") to the size of the dots used, such irregularities are perceived only as a reduction of the quality of the image presented.
The impact dot matrix printer, wherein images are produced using a typewriter style inked ribbon and a plurality of wire "hammers" which impact the ribbon, thus causing the ribbon to deposit ink upon the medium, embodied the first large scale application of this principle in the field of computer hardcopy output. A later development has been the ink-jet printer. Ink-jet printers utilize the same dot matrix concept as do impact dot matrix printers. However, instead of depositing ink upon the medium by means of a hammer impacting an ink impregnated ribbon, ink-jet printers eject droplets of ink from a printhead onto the print medium. Each droplet, upon striking the medium, forms a small dot.
Ink-jet printers have several advantages over impact printers in that they can be made to operate more quietly, faster, and more reliably (since there are far fewer moving parts). Also, ink-jet printers are more readily adaptable to applications requiring multi-colored print images, since they can easily be designed to incorporate nozzles or sets of nozzles for more than a single color of ink. In fact, it has become common to utilize four sets of nozzles in such applications, for the colors of cyan, magenta, yellow, and black. Cyan, magenta, and yellow are referred to as the subtractive primary colors. Additional color combinations can be produced by causing the printer to deposit multiple dots of different colored inks in the same locations, thus effecting a perceived mixing of the colors to form what are known as secondary colors.
While it is theoretically possible to produce a multitude of different colors and shades of colors by overlaying three or more droplets of ink on each individual potential dot location ("pixel"), practical limitations such as the ability of the medium to readily absorb the liquid ink carrier, the available drying time between successive depositing of droplets, and the fact that the semi-opaque character of dried ink droplets results in a diminishing effect for each successively applied droplet have resulted in a practical limitation of a maximum of two droplets per pixel in most applications. Thus, using the above described scheme, a maximum of eight colors are available per pixel, these being:
white (no ink deposited),
black,
yellow,
cyan,
magenta,
red (magenta and yellow),
green (yellow and cyan), and
blue (cyan and magenta).
Obviously, a scheme for producing a multitude of different colors and shades (lighter and darker variations of colors), without violating the practical limitation of a maximum of two droplets of ink per pixel, is desirable. However, methods for doing this that have been developed have proven to be less than totally satisfactory since they have either not included provision for a drying interval between applications of ink to contiguous pixel locations or groups of pixel locations, or they have not addressed the problem of color banding. Further, prior art methods have not provided a rational pattern for laying down the various inks wherein optimal image quality may be obtained.
A number of means for causing the ink to expel from the printhead in ink-jet printers have been tried with varying degrees of success. These include electrostatic means (wherein the ink is either repelled or accelerated by means of electrostatic repulsion or attraction), and thermal means (wherein an individual droplet of ink is rapidly heated and vaporized and is effectively boiled out of a nozzle). Since the present invention pertains to the allocation and sequence of application of inks to pixel locations of a print medium, it is equally applicable to any of these various methods for physically depositing ink within the pixels.
An ink-jet printer must also include a mechanism for positioning an ink-jet nozzle in a proper location over the print medium and for then causing the nozzle to deposit ink upon the medium at that location. This is generally accomplished, under computer control, by providing a means for moving the medium, by regular increments, past a printhead location. After each such incremental advancement of the medium, a printhead containing one or more ink nozzles is moved across the medium in a direction perpendicular to the direction of the advancement of the medium. At each of a plurality of incremental positions along this perpendicular printhead path, each of the nozzles contained therein is caused by the computer to either eject an ink droplet or to refrain from doing so. By repeating this process, every potential pixel location on the medium may be addressed.
Due to the extremely small sizes of the ink nozzles utilized, a plurality of such nozzles may be contained on a printhead. Further, since it is desirable to reduce the number of perpendicular traversals of the printhead across the medium, it is desirable to include as many nozzles in the printhead as is practical. It is also desirable, in order to obtain complete coverage of the medium, to have overlapping coverage of ink dots, as depicted in FIG. 6. Therefore, it is not practical to have all nozzles contained within a single column. This does not present a problem, however, since the nozzles may be staggered as shown in FIG. 7. When the printhead is being moved in the direction shown by the arrow, a desired print pattern such as that shown in FIG. 8 may be created under computer/processor control by staggering the timing of the firing of nozzles as they pass over the medium. That area of the medium which may be imprinted with a single pass of the printhead is referred to as a swath.
The above described arrangement of nozzles is applicable for each of the several colors of ink which may be employed in a color printer. Columns of nozzles for each color may be arranged in any of several different manners including parallel to each other on a common printhead, parallel to each other on individual printheads, or consecutively on either a common or individual printheads.
While any of these physical arrangements of nozzles is workable, as are many other potential arrangements, they also each present potential pitfalls to the goal of creating high quality multi-colored or multi-shaded images. First, as discussed previously, if multiple colors are to be derived from the four colors of ink used (including black), with the limitation that a maximum of two colors may be applied per pixel, only eight colors (including black and white) can be produced. This is far less than a desirable full spectrum of colors and shadings. Further, problems associated with all ink-jet printers, such as "bleeding" of ink from one pixel into another, are compounded when multiple colors or shadings are desired because when more ink is used, bleeding and other migrations of wet ink are more likely to occur, and because inadvertent intermixing of colors or shadings may produce a particularly objectionable product.
"Bleeding" of ink from one pixel into another is most likely to occur when the ink in both pixels is simultaneously wet. The surface tensions of two contiguous droplets of ink may yield to a combined tension encompassing both droplets, thus allowing the ink to flow between the pixels whereon these droplets are located. Various schemes have been utilized to provide a drying time between applications of ink to contiguous pixel locations. U.S. Pat. No. 4,748,453 issued to Lin et al. teaches an example of one of these application methods. The Lin patent describes a method wherein only half of the pixels in a swath are printed at one pass of a printhead. The printhead is then moved over the same area again, with the other half of the pixels in the swath being printed on that second pass. This method reduces the number of simultaneously wet contiguous pixels somewhat, but it does not increase the number of colors or shadings available, and it does not address the additional problem of "banding".
Banding is a series of noticeable belts or bands across the print medium. These are commonly caused by mechanical misalignment of printer parts such as step error, drop volume variations, or nozzle directionality. Step error is the overshoot or undershoot of the medium as it is advanced. For example, if a printer is designed to advance the medium 0.167 inch after each swath is printed, but because of variances in part and/or assembly tolerances it actually advances 0.170 inch, there would be a 0.003 inch step error. This means that there would be a 0.003 inch unprinted band across the paper between each swath. Drop volume variations may be caused by tolerance variations in nozzle sizes or other irregularities. Nozzle directionality refers to variations in the precise angles at which ink droplets are ejected from the various nozzles. Any of these irregularities can cause a banding problem. The combined method of the present invention teaches a way to avoid this problem.
Therefore, while methods have been developed to reduce unwanted migration of inks between contiguous pixel locations, to the inventors' knowledge, no prior art method has provided a way to reduce migration while also eliminating the "banding" problem. Furthermore, no prior art method for producing multi-colored or multi-shaded images has provided a way to prevent banding while reducing unwanted ink migration.